The present production and global consumption of plastics (especially for packaging) is continuously increasing, leading to an inefficient waste management system. One of the major environmental risks is the slow rate of its degradation or even the non-biodegradability of some organic compounds in real life systems. Therefore, green additives and adequate processing of the packaging materials are needed to intensify the plastic biodegradation under natural conditions. In this work, commercial grade low-density polyethylene (LDPE) with 1% rosemary (Rosmarinus officinalis) extract was used for the preparation of composite films, without further treatment or purification. The biodegradability studies were carried out by incubating the unmodified and modified composites with Aspergillus Niger. The thermal runs were performed on a SETARAM microDSC 7 evo differential scanning calorimeter within the 50–120^oC temperature range, at heating rates of 0.3, 0.4, 0.5, 0.6 and 1 ^oC min^-1, in nitrogen atmosphere with a flow rate of 50 mL min^-1. The results show that the slight increase in the crystallinity degree after microbial attack can be caused by consumption of the amorphous part of polymer by microorganisms. Peak melting temperatures are practically unaltered within experimental errors. Based on microcalorimetric results it can be concluded that rosemary extract can be used for increasing the biodegradability of polyethylene films via reducing of its crystallinity degree.